Method and apparatus for producing clear ice under quiescent conditions



1967 J. E. BURROUGHS ET AL METHOD AND APPARATUS FOR PRODUCING CLEAR ICE UNDER QUIESGENT CONDITIONS Filed Sept 50 1965 IIIIIIII IIIIIIIIIIII I [big (III/III!!! 5 Y R w man R 0 0 T WW T W A wk.

WW im Y B w United States Patent 3,318,105 METHOD AND APPARATUS FUR PRUDUCING CLEAR ICE UNDER QUEESEENI UGNDITIQNS James E. Burroughs, Mount Prospect, and James K. Nelson, Des Plaines, Ili., assignors to Borg-Warner 0rporation, Chicago, lift, a corporation of Illinois Filed Sept. 30, 1965, Ser. No. 491,574 6 Claims. (Cl. 62*71) This invention relates to the production of clear ice under quiescent conditions, and more particularly to an improved method and apparatus for freezing water in a conventional, domestic refrigerator freezing compartment such that the ice produced is clear, crystalline, and substantially free from fractures and dissolved gases and minerals which are responsible for a cloudy appearance.

It is a principal object of the invention to provide an economical and efiicacious process for making clear, crystalline ice without requiring turbulent conditions at the ice-water interface while freezing takes place.

Another object of the invention is to provide a novel and improved process for producing ice under quiescent conditions wherein the freezing process is conducted in such a way to avoid the occlusion of unfrozen (liquid) water in the ice block or cube which eventually expands to form fractures therein.

Another object of the invention is to provide an improved process in accordance with the foregoing objects wherein freezing is conducted while maintaining a free path for the escape of dissolved gases which may evolve during the freezing process.

Another object of the invention is to provide apparatus suitable for carrying out the aforementioned process which may be used in conjunction with a conventional domestic refrigerator.

Still another object of the invention is to provide apparatus capable of carrying out the aforementioned process which is suitable for use in an automatic ice maker.

Additional objects and advantages will become apparent from reading the following detailed description.

Most commercial ice making processes capable of producing clear ice are usually conducted with a continuous flow of water against a cold surface, such as by passing liquid water through a plurality of chilled tubes or by spraying it against a refrigerated ice mold. Clear ice can be obtained from such apparatus because the water is frozen under conditions which insure that substantial agitation and turbulence exists at the liquid solid interface to avoid freezing of dissolved gases and minerals into the solid phase.

Heretofore, many attempts have been made to incorporate the techniques for producing commercial ice in a domestic refrigerator or freezer; but these attempts have not met with significant success because they have tended to rely primarily on scaled-down versions of the means used in commercial ice makers. These mechanical features, which include, for example, means for agitating or continuously circulating water during the freezing period, are simply too expensive, cumbersome, and complicated for consumer acceptance. Moreover, the con tinuous flow technique necessarily requires that the unfrozen water into which the minerals are concentrated during the freezing period be discarded intermittently, usually during the harvesting cycle. Unless a drain system is provided with the refrigerator, this would require manual removal of the waste water at annoyingly frequent intervals; and even if a drain system is used, the choice of location for the refrigerator or freezer is restricted to being near a sink or other waste water outlet.

In the conventional refrigerator, ice cubes are made in a batch process by charging water into a freezing tray and leaving it in a quiescent or stationary condition durice ing the entire freezing period. It has long been recognized that without means to agitate the charge of Water within the mold cavities, the minerals and gases in the water cause the ice to be frozen with a cloudy appearance which is undesirable.

He-retofore, it has been proposed that the raw water be de-aerated and de-mineralized to prevent bubbles or dissolved minerals from being frozen in sit-u. Examples of such treatment are found, for example, in United States Patent 2,528,875, issued to A. C. Embshoff on Nov. 7, 1950. While de-mineralization is economically feasible, it is practically impossible to keep water deaerated for any period of time. Moreover, we have found that additional factors are responsible for contributing to the undesirable appearance of ice frozen under quiescent conditions. These factors, and the manner of avoiding them, will now be described.

Referring now to the drawings:

FIGURES 1A-1H illustrate, in diagrammatic form, the sequential freezing of ice Within a conventional ice tray;

FIGURES 2A-2E represent, in diagrammatic form, the various stages of ice formation as the process of the present invention is conducted in one type of mold;

FIGURE 3 is a cross-sectional view of an improved ice mold for producing clear ice which is suitable for use in the freezing compartment of a domestic refrigerator;

FIGURE 4 is a cross-sectional view showing a modified form of an ice mold which includes a small heater; and

FIGURE 5 is a cross-sectional'view of an ice mold which may be used in an automatic ice maker of the type which continuously freezes, harvests, and transfers the frozen cubes to a remote storage zone.

It would be desirable to analyze the progression of ice formation in a conventional ice tray by referring first to FIGURES 1A to 1H. The typical prior art ice tray It may be of any suitable type, but for purposes of this example, it is shown as being an all-metal tray including a pan 11 and a divider 12 separating the pan into a plurality of cavities 14. Both pan 11 and divide-r 12 are usually fabricated from a material, such as aluminum, which exhibits good thermal conducting properties; although many trays are made from rubber or plastic to facilitate quick release of the frozen cubes and still other types of trays are made with a metal pan and a plastic or rubber divider.

Initially (FIGURE 1A), when the ice is charged into the tray, the Water is chilled to remove its sensible heat until the freezing point is reached. After approximately fifteen minutes, as shown in FIGURE 1B, a thin layer of ice It begins to develop along the inside wall of the pan 11 and also at Ma along the wall portions provided by the divider 12. If a metal divider is used, the ice formation on the divider will be more rapid; but in any case, after approximately fifteen to thirty minutes, the ice will begin to form into cup-shaped shells. As the thickness of the shell increases (FIGURE 1C), the impurities in the water are retained in the unfrozen, liquid phase which becomes more concentrated with such impurities. After approximately forty-five minutes (as shown in FIGURE 1D), the convective heat transfer between the upper surface of the water and the cold air above the ice tray is effective to form a thin film of ice 16b which entraps or occludes a mass of liquid water 17 containing a relative high concentration of impurities-minerals and gases-ejected from the ice shell during its formation. As the ice blocks continue to freeze (FIGURE IF), the pressure generated by the expansion of the liquid center forces the frozen upper layer to yield, creating a bulge, and tiny fissures 18 within the block. Eventually (FIGURE 1G), the shell is ruptured, forming myriad, striated fractures which, together with the bubbles and mineral impurities, make the cubes or blocks cloudy and generally transluscent in appearance, as illustrated in FIGURE 1H.

The improved process of the present invention contemplates freezing of ice in a staionary mold in such a way that the ice is formed in thin layers, said layers advancing in substantially unidirectional fashion while maintaining an unobstructed path through liquid water in said charge between each particle of water in the charge and the ambient atmosphere until the ice is in condition to be harvested, whereby the occlusion of a body of liquid water in ice is avoided. By preventing the formation of an entrapped pocket of liquid, the small cracks and fissures created by subsequent expansion of said liquid are eliminated; and the dissolved gases can be freely released at the upper surface of the charge.

One manner of practicing the invention, which is especially adapted for use in a conventional refrigerator, is shown in FIGURES 2A-2E. The special mold 20 comprises means defining one or more freezing cavities 22, said means including a bottom wall 24 formed from a material having a very high k-factor or coefficient of thermal conductivity and side walls 26 made of material having a low k-factor. The upper portion of the mold may remain open if the air temperature above the mold is relatively high; or it can be provided with insulation or other means, to be described in more detail below, for retarding the formation of ice on the upper surface of the charge.

The bottom wall of the mold 24 is preferably maintained in engagement with a refrigerated surface (not shown) such as the bottom of the freezer box in a refrigerator. As heat is rapidly abstracted through the bottom wall 24 (FIGURE 2A), the charge of water therein begins to approach the freezing temperature. A thin layer of ice 28 first begins to form on the inside surface of the bottom wall 24; and since the side walls 26 and the zone above the charge of water are thermally isolated from the heat abstraction surface, the ice builds up from the bottom and advances unidirectionally toward the upper surface of the charge.

Most of the gas bubbles in the liquid phase, as indicated in FIGURE 2D at 29, are pushed upwardly toward the surface of the charge by the advancing wall of ice 3% and are expelled at the upper surface. When the charge is completely frozen, the cubes (FIGURE 2E) are clear, crystalline, and substantially free from the cracks and fissures caused by an occluded liquid center which would be formed during freezing in a conventional ice tray.

The mold or container to be used for producing the clear ice must possess dimensions and characteristics which will permit the unidirectional freezing of the wa- Iter. The material from which the mold or container is formed must have properties which are compatible with the temperature of the surroundings in which the freezing is accomplished. These requirements indicate that the thermal conductivity of the sides of the container be much lower than the bottom wall if freezing is to be performed from the bottom to the top of the container. 1f the freezing is to be performed from one side to the other, it is clear that the thermal conductivity of one side wall must be higher than that of the bottom wall and other side walls.

The unidirectional freezing cavities may be made from a single material of construction or a combination of materials, as long as such materials possess those characteristics which are acceptable for making consumable ice. Materials of low conductivity and suitability include plastics, waxes, rubbers, alumina, ceramics, glasses, porcelains, woods, various metal alloys, and combinations of these, as well as numerous other ma- Cir lterials. Materials of high thermal conductivity include metals such as aluminum, steel, brass, nickel, silver, and the like cermets, and plastics loaded with metallic fillers to increase their thermal conductivity properties. Therefore, the essential requirements for the container is that it possesses Optimum thermal characteristics for unidirectional freezing of the water which is contained in it.

The time required to harvest the ice from such containers is related to the geometry or design thereof. By varying the thickness of the bottoms and sides of the containers, the harvest time can be increased or decreased. Also, a variation in the temperature of the freezing compartment will afiect the harvesting rate.

The ice mold construction shown in FIGURE 3 is similar to the one described above, but includes a cover member 4% for more effective shielding of the upper surface of the mold. The cover member is also formed of a material having a low k-factor and is preferably provided with a small opening 41 to afford communication between the zone above the surface of the charge and ambient atmosphere to permit the evolved gases to freely escape. In some circumstances, this can be accomplished by a loose fit of the cover member on the mold.

In some cases, it is quite difficult to shield the upper surface of the Water charge from the cold air above. This is particularly true in freezers which are equipped With a blower or other means for effecting forced circulation of the air through the freezing compartment. If this is the situation, then it is possible to prevent the freezing over of the upper surface of the charge by providing heating means, preferably carried by the cover member, for directing thermal energy on the surface of the water during freezing. As shown in FIGURE 4, a heating element 43 is carried by the cover member, said heating member comprising a relatively low wattage, resistance element which is adapted to be connected to a power supply and energized during the freezing period. The amount of heat directed onto the upper surface of the Water is just sutficient to prevent freezing of said surface.

The method described in connection with FIGURES 2A-2E may also be performed in an apparatus which is suitable for use in an automatic ice maker. This freezing mold, shown in FIGURE 5, does not include the lower wall portion of thermally conductive material as in the case of the mold shown in FIGURES 2A-2E. Rather, the freezing surface onto which the ice is frozen takes the form of a metallic finger which extends downwardly into the charge of water to be frozen and is spaced from the wall of the mold cavity.

The mold, generally designated at 50, comprises a cup-shaped receptacle formed of a material. having a relatively low coefficient of thermal conductivity and a plurality of heating elements 51 embedded in the mold along the surface of the cavity 52. A probeor finger element 54 is supported above the mold by a bracket 55 which may be provided with a heater element 55 to facilitate harvesting. The finger 54 is preferably made of metal or other heat con-ducting material, and extends downwardly into the cavity and in spaced relation from the bottom and side walls of said mold cavity. When the water to be frozen is charged into the cavity, the heating elements 51 are energized to direct thermal energy along the surface of the mold cavity in contact with the water. Since finger element 54 is made from a material having good heat conducting properties, the water in the charge will be chilled locally of the freezing surface provided by the finger. Initially, a thin film of ice will begin to form on the finger and the ice Will build up layer on layer until the cubes 5'7 are in condition for harvesting. The heat generated by the heating elements is sufficient to maintain a body of liquid water 58 surrounding the ice adhering to the finger and thus prevent the formation of a layer of ice bridging the gap between the finger and the side walls of the mold. It can be seen, therefore, that the method as practiced by the modified form of apparatus is identical with that described in connection with the mold of FIGURES 2A-2E because all during the freezing period a path through liquid water is maintained between each particle of liquid water in the mold and ambient atmosphere to prevent the formation of an occluded pocket of liquid water in ice.

It should be understood that the mold cavity shown in FIGURE 5 represents only one of many which would be normally formed in a grid made up of a plurality of ice molds separated by the side wall defining portions or individually suspended by brackets within the freezing chamber. In response to suitable controls which determine the point at which the ice is in condition for harvesting, the fingers are lifted out of the mold and transported laterally so that the adhering ice can be discharged at a remote location to a storage bin. In a preferred embodiment, the support member for the fingers would be provided with a heater which would be energized during the harvesting cycle such that the heat would be conducted to the fingers to facilitate the release of the cubes therefrom.

A preferred manner of practicing the invention also includes the use of de-ionized, substantially mineral-free water because tap water or softened water contains a high concentration of dissolved salts, one of the major causes of turbid or cloudy ice. While distilled, de-aerated Water would be ideal, the cost of water treated in this fashion is economically prohibitive for most purposes. The de-ionized water does not have to be completely deaerated since the dissolved gases are continuously moved toward the upper surface and finally ejected out of the liquid phase by the advancing ice layer.

While this invention has been described in connection With certain specific embodiments thereof, it is to be understood that this is by way of illustration and not by way of limitation; and the scope of this invention is defined solely by the appended claims which should be construed as broadly as the prior art will permit.

What is claimed is:

1. A method of producing clear ice under quiescent conditions from demineralized water comprising the steps of charging demineralized water into means defining a cavity; providing a freezing surface in contact with said water to be frozen; cooling said water locally of said freezing surface so that ice is frozen thereon and proceeds to build up layer on layer; and concurrently supplying thermal energy, by heating means, in a localized zone of said water charge to retard freezing along the upper surface thereof and to maintain an unobstructed path through liquid Water in said charge between each particle of water in the charge and the atmosphere above said charge until the ice is in condition to be harvested, whereby the occlusion of a body of liquid water in ice is avoided.

2. A method as defined in claim 1 wherein said freezing surface is constituted by a portion of said cavity defining means.

3. A method as defined in claim 1 wherein said freezing surface is independent of said cavity defining means,

4. A method as defined in claim 3 wherein said freezing surface comprises a heat conductive member extending into the upper portion of said charge and spaced from said cavity defining means.

5. A method of producing clear ice under quiescent conditions from demineralized water comprising the steps of charging a predetermined quantity of demineralized Water into means defining a cavity; providing a freezing surface in contact with said charge of water, said freezing surface comprising a heat conductive member extending into the upper portion of said water and spaced from the surfaces of said cavity in contact with said Water; cooling said water locally of said freezing surface so that ice is frozen thereon and continues to build up in substantially unidirectionally advancing layers; supplying thermal energy at said cavity surfaces in contact with said water to maintain a body of liquid water between said ice and said surfaces until said ice is in condition for harvesting, said body of liquid water providing an unobstructed path between each particle of water for said charge and the ambient atmosphere to prevent the occlusion of a body of liquid water in ice; removing said heat conductive member, together with the ice adhering thereto; and thereafter releasing the body of ice to a remote storage zone.

6. An ice mold comprising a bottom wall formed of a material having a relatively high coefficient of thermal conductivity, side walls formed of material having a relatively low coetficient of thermal conductivity, and a cover member substantially enclosing the chamber defined between said bottom and side walls, said cover member also being formed of a material having a relatively low coefficient of thermal conductivity; heating means carried by said cover member adapted to direct thermal energy on the upper surface of water charged into said mold; and fluid passage means in said cover member interconnecting the zone above water charged into the mold with ambient atmosphere outside said mold to provide an unobstructed path for the escape of evolved gases.

References Cited by the Examiner UNITED STATES PATENTS 529,345 11/1894 Church.

625,447 5/ 1899 Humes 62-66 X 2,528,875 11/1950 Embshoff 62-66 2,704,928 3/1955 Curry 46 X 2,775,099 12/1956 Brown 62-352 X References Cited by the Applicant UNITED STATES PATENTS 1,817,545 8/1931 Copeman. 1,931,053 10/ 1933 Berkeley. 2,114,642 4/1938 West. 3,143,866 8/ 1964 Frobbieter.

ROBERT A. OLEARY, Primary Examiner. W. E. WAYNER, Assistant Examiner, 

1. A METHOD OF PRODUCING CLEAR ICE UNDER QUIESCENT CONDITIONS FROM DEMINERALIZED WATER COMPRISING THE STEPS OF CHARGING DEMINERALIZED WATER INTO MEANS DEFINING A CAVITY; PROVIDING A FREEZING SURFACE IN CONTACT WITH SAID WATER TO BE FROZEN; COOLING SAID WATER LOCALLY OF SAID FREEZING SURFACE SO THAT ICE IS FROZEN THEREON AND PROCEEDS TO BUILD UP LAYER ON LAYER; AND CONCURRENTLY SUPPLYING THERMAL ENERGY, BY HEATING MEANS, IN A LOCALIZED ZONE OF SAID WATER CHARGE TO RETARD FREEZING ALONG THE UPPER SURFACE THEREOF AND TO MAINTAIN AN UNOBSTRUCTED PATH THROUGH LIQUID WATER IN SAID CHARGE BETWEEN EACH PARTICLE OF WATER IN THE CHARGE AND THE ATMOSPHERE ABOVE SAID CHARGE UNTIL THE ICE IS IN CONDITION TO BE HARVESTED, WHEREBY THE OCCLUSION OF A BODY OF LIQUID WATER IN ICE IS AVOIDED. 